Steam flooding oil-bearing limestone strata



i led? tinned rates Patent 3,405,761 STEAM FLOODING OIL-BEARING LIMESTONE STRATA Harry W. Parker, Bartlesville, Okla, assignor to Phillips ietroleum Company, a corporation of Delaware No Drawing. Filed May 12, 1967, Ser. No. 637,920 9 Claims. (Cl. 166-11) ABSTRACT OF THE DISCLOSURE Oil-bearing limestone strata are treated with steam and carbon dioxide sequentially injected thru a common tubing string or simultaneously or separately injected thru dual strings to effect mixing of CO and condensate principally in the stratum to effect dissolving of limestone to enhance oil production without corrosion problems in the well equipment.

The invention relates to an improved method of steam flooding an oil-bearing limestone stratum.

Up to this time, steam has been used principally to treat sand strata to stimulate oil production by direct drive or hutf-putf techniques. Since most of the oil heretofore produced has been from sand strata, such oil fields will soon be depleted and the petroleum industry will find it necessary to obtain much of future oil production from l'imestone strata. It is now estimated that limestone strata contain about 70 percent of the worlds available oil supply. In steam drive in any stratum, one of the problems encountered is that of low-injection rates and high pressures required to force the steam and resulting fluid thru the formation. Any method which increases the permeability of the stratum and enhances flow rate of steam without excessive pressures is an advance in the art.

Accordingly, it is an object of the invention to provide an improved method of steam flooding an oil-bearing limestone stratum to improve the recovery of oil therefrom. Another object is to increase the permeability and rate of flow of fluids thru a limestone stratum containing displaceable oil. A further object is to provide a method of utilizing a combination of steam and CO in a steam drive which avoids corrosion problems from resulting carbonic acid in the well equipment. Other objects of the invention will become apparent to one skilled in the art upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises injecting steam into an oil-bearing limestone stratum thru a Well therein to displace oil therefrom into a production well, separately injecting CO into the stratum thru the well used for injecting steam so as to mix CO with resulting steam condensate within the stratum thereby forming carbonic acid and dissolving limestone from the surrounding stratum, displacing fluids resulting from the injection steps into said production well, and recovering the produced fluids from the production well. The amount of CO injected lies in the range of l to weight percent, preferably, 2 to 6 weight percent based upon the weight of the steam injected.

'The method of the invention is applicable to but? and puff steam injection wherein steam is injected into a well until a substantial amount of heat has been added to the stratum adjacent the well, after which there is a substantial soaking period followed by release of pressure at the injection well to allow fluids, including heated oil, to flow into the well thru which the steam was injected, thereby using this well as a production well. This sequence of steam injection, soaking, and production is repeated as many times as is practical and economical. In each injection phase of the process, the amount of steam injected is Ciilll'uil in the range of about 1000 to 7000 pounds per foot of pay thickness.

The method of the invention is also applicable to a direct steam drive process wherein steam is injected thru a central well with production of fluids thru any number of ring wells. Steam may also be injected thru a line of injection wells flanked on either side by lines of production wells. In any event, the amount of CO -injected is in the range of l to 10 weight percent-based upon the weight of the injected steam.

In one embodiment of the invention, the injection of steam and CO are effected sequentially either thru the same tubing string or thru separate strings. In accordance with another embodiment of the invention, the steam and CO are injected thru separate tubing strings extending to stratum level, either sequentially or simultaneously. In any event, CO and steam are not injected in admixture because of the corrosion difliculties in the well apparatus. In practicing sequential injection of steam and CO thru the same tubing string, it is preferred to inject a purge gas free of moisture between slugs of steam and CO to prevent corrosion in the well apparatus and to assure mixing of the CO and steam condensate substantially completely in the surrounding stratum. Nitrogen or air may be use-d for this purpose.

The injected CO dissolves in the steam condensate which forms carbonic acid and the resulting solution of acid is effective in dissolving calcium carbonate from the limestone stratum which is in the form of calcium bicarbonate. Channels are developed in the vicinity of the in jection well for a net increase in injectivity and oil flow,.

particularly in the huff and puff steam injection operation in which the oil flows back into the injection well after the soaking period.

In accordance with one procedure, steam is injected thru one tubing stream extending into the stratum to be produced and CO is injected thru a second tubing string extending to the lowermost level of the stratum with a packer on this second tubing string near the bottom of the stratum thru which carbon dioxide injection is limited to the lower section of the stratum below the steam face. This assures mixing of the CO with the steam condensate entirely within the stratum. It is also feasible to inject CO into an intermediate level of the stratum and steam at levels above and below this intermediate level. Likewise, the CO may be injected into the stratum at a level near the top thereof while steam is injected across the remaining face of the stratum.

To illustrate the invention, percent quality steam at a temperature of about 500 F. is injected across the upper 17 feet of a 20 foot thick limestone pay zone while injecting CO into the lower three feet of pay thru a separate tubing string packed off from the steam injection section of the pay. Fifteen hundred pounds of steam per foot of pay are injected over a period of about ten days without production from surrounding wells. The simultaneous injection of CO amounts to 60 pounds per foot pay, making a total of 1200 pounds of CO and 30,000 pounds of steam. After a soaking period of about one week, the injection well is opened to production and the solution of CO in the steam condensate has the effect of acidizing the surrounding stratum with increased production of oil. Another effect is the improvement of steam injection rates at a given steam pressure.

Obviously, the steam temperature can be varied and will usually be in the range of about 450 to 700 F.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

'l 1 l t I claim:

1. A method of improving oil production from an oilbearing limestone stratum which comprises the steps of:

(a) injecting steam into said tratum thru a well therein to displace oil therefrom into a production well therein;

(b) separately injecting CO in the range of 1 to 10 wt. percent of the steam into said stratum thru the well used for injecting said steam so as to mix CO with resulting steam condensate thereby forming carbonic acid and dissolving limestone from the surrounding stratum;

(c) displacing fluids resulting from steps (a) and (b) into said production well; and

(d) recovering the produced fluids;

2. The method of claim 1 wherein injection and production are effected thru a common well, using a substantial soaking period between steps (b) and (c).

3. The method of claim 1 wherein injection and production are effected thru separate wells.

4. The method of claim 1 wherein the weight of the CO injected is in the range of 2 to '6 wt. percent.

5. The method of claim 4 wherein steps (a) and (b) are effected sequentially.

6. The method of claim 4 wherein steps (a) and (b) References Cited UNITED STATES PATENTS 2,862,558 12/1958 Dixon 16611 X 3,221,813 12/ 1965 Closmann et al 166-40 X 3,259,186 7/1966 Dietz 16611 3,292,702 12/ 1966 Boberg 16640 3,333,637 8/1967 Prats 166-40 3.354958 11/1967 Parker 16640 3,358,759 12/1967 Parker 166-40 X 3,360,044 12/1967 Lange 166--40 X STEPHEN I. NOVOSAD, Primary Examiner. 

